The present invention is generally directed to an RF power amplifier with an automatic gain control system for use in amplifying an RF input signal and is more particularly directed toward frequency and amplifier failure compensation.
RF power amplifier systems are known in the art for use in amplifying RF signals for broadcasting purposes, including radio and television. Such power amplifiers may be employed in the broadcasting of either analog television signals, known as the NTSC, PAL, SECAM format, or digital signals, sometimes known as DTV format. When employed in television broadcasting, the frequency bandwidth for the television signals is 6 MHz. The television channels will be in the UHF signal range from approximately 470 to 860 MHz.
The RF input signal to such a power amplifier is obtained from an RF exciter and, for example, this may take the form of a modulated RF carrier within a frequency band of 470 to 860 MHz with the bandwidth of any one channel being on the order of 6 MHz. This RF input signal may have a magnitude on the order of 20 milliwatts. This input signal is increased in magnitude to a much higher level such as on the order of 400 watts which represents a gain on the order of 43 dB.
These power amplifier systems are expected in many situations to be constantly operating so that a radio or television station employing such a power amplifier system may continuously broadcast.
Broad band RF power amplifiers are often required to have a flat gain over the specified frequency range. Usually, an automatic gain control (AGC) closed-loop system is used to control the gain of an amplifier. Most AGC closed-loop controllers are analog, however, the AGC can be digital by using a microprocessor for complex algorithms. A proportional integral derivative (PID) algorithm based AGC closed-loop controller can be used to control the gain of a power amplifier. A PID controller for AGC, which includes both analog and digital components, can automatically adjust the gain, compensating for the variations caused by the temperature changes or a power supply voltage change. It has been widely used in industrial control.
The regular closed-loop AGC control system of a RF power amplifier operating at nominal frequency can be described in the block diagram of FIG. 10 and Equation A.                                                         P              out                        ⁡                          (                              j                ⁢                                  xe2x80x83                                ⁢                ω                            )                                                          P                              i                ⁢                                  xe2x80x83                                ⁢                n                                      ⁡                          (                              j                ⁢                                  xe2x80x83                                ⁢                ω                            )                                      =                                            G              I                        ·                                          G                C                            ⁡                              (                                  j                  ⁢                                      xe2x80x83                                    ⁢                  ω                                )                                      ·                                          G                P                            ⁡                              (                                  j                  ⁢                                      xe2x80x83                                    ⁢                  ω                                )                                                          1            +                                          G                O                            ·                                                G                  C                                ⁡                                  (                                      j                    ⁢                                          xe2x80x83                                        ⁢                    ω                                    )                                            ·                                                G                  P                                ⁡                                  (                                      j                    ⁢                                          xe2x80x83                                        ⁢                    ω                                    )                                                                                        Equation        ⁢                  xe2x80x83                ⁢        A            
The ideal RF sensor characteristics are:
Vin=GI*Pin xe2x80x83xe2x80x83Expression (1) 
Vout=GO*Pout xe2x80x83xe2x80x83Expression (2) 
Here,
The regular AGC closed-loop controller in FIG. 10 can not compensate the gain errors due to the variation of the characteristics of components, which are not inside the closed-loop or in the feedback loop of the control system. To make the AGC loop work properly and provide constant gain over wide frequency range, the characteristics of the input and output power sensors should be frequency-independent. The same power level should generate the same response from the sensor at all frequencies. Consequently, the accuracy of the closed-loop controller maintaining the gain constant over frequency will depend on the flatness of the power sensor frequency response.
The power sensors are not perfectly flat over frequency, which limits the possibility for the controller to maintain constant gain over frequency. In order for an RF amplifier system to operate at a given frequency, the power sensors have to be tuned by means of variable capacitors and/or variable resistors. Hence, they can only be used at a fixed frequency, if operated without any other compensation.
The amplifier itself can contain several sub-amplifiers, which work in parallel. The failure of one of them changes the overall gain. A simple AGC controller would restore the gain level by increasing the RF drive level. That would overdrive the working sub-amplifiers and sacrifice the overall performance. The degradation in performance can not be adjusted nor compensated by the AGC, since the amplifier is driven into non-linear working region.
A cancellation technique can be used to correct gain variations by correcting the variation of the input and output signals from the RF sensors, which are due to the frequency response of these sensors, before the signals are used for the PID AGC algorithm. A similar technique can be used to compensate the gain variation caused by sub-amplifier failure.
In accordance with one aspect of the present invention, a power amplifier system is provided for use in amplifying an RF input signal at a given operating frequency within a known range of frequencies. The system includes a signal modulator that receives and modifies an RF input signal and provides therefrom a modified first signal. At least one power amplifier is provided that receives and amplifies the modified first signal. A first power detector detects the input signal and provides therefrom an average input power signal representative of the average input power thereof. A second power detector is connected to the output of the amplifier and provides an output average power signal representative of the average output power thereof. A controller adjusts the magnitude of the average input power signal and adjusts the magnitude of the average output power signal as a function of the operating frequency. The controller compares the adjusted input average power signal with the adjusted output average power signal and controls the modulator in accordance therewith.
In accordance with a more limited aspect of the present invention, the power amplifier is comprised of a plurality of sub-power amplifiers connected together in parallel and wherein the controller monitors the operation of the sub-power amplifiers for determining whether any of the sub-power amplifiers exhibits a faulty condition and adjusting the magnitude of the output average power signal in accordance therewith.